Recently, as illumination light sources in luminaires, more and more incandescent lamps and fluorescent lamps are being replaced by energy-saving long-life light sources, for example, light-emitting diode (LED). Also, new illumination light sources, for example, electro-luminescence (EL) and organic light-emitting diode (OLED), are being developed. Since light output of these illumination light sources depends on the value of a current flowing therein, a lighting circuit capable of controlling the current value supplied is necessary in order to light and dim the illumination light source. For example, a power supply device which lights a light-emitting diode needs to control the current over a broad range from a maximum current to fully light the light-emitting diode to a minimum current value of zero to turn off the light.
However, at the time of deep dimming which is close to light-off state, the current value becomes very small and detection error increases. Also, since the properties of light-emitting diode varies depending on manufacturing variance, temperature dependence and the like, smooth dimming from full lighting to deep dimming and then to light-off cannot be carried out in some cases.